Complex transmission schemes as used in commonly known transmission systems like mobile communication, radio broadcasting or satellite transmission are well known in the fields of information transmission. Therein, information is complexly transmitted from a transmitter to a receiver locally displaced to each other. In the transmitter, a complex modulator multiplies an information signal with two linearly independent signals. The result is a complex modulation signal including two real signal components. Next, the complex modulation signal is modulated onto a high-frequency signal and transmitted from the transmitter to the receiver via an antenna. In the receiver, a complex demodulator reconstructs the information signal from the complex modulation signal based on a suitable demodulation technique. The complex and high-frequency modulation can also be performed in one single step. Complex transmission schemes were introduced in form of the single side band modulation, which was especially used for information transmission in the high-frequency spectrum (HF-spectrum).
The linearly independent signals used for generating the complex modulation signal are usually chosen as sinus-signal and cosinus-signal. Since these signals comprise the same amplitude but a phase-difference of 90°, the complex modulator and demodulator can be described with complex numbers facilitating the overall calculation and design process. The sinus-signal is known as in-phase signal, wherein the cosinus-signal is known as quadrature signal. The main advantage of complex transmission schemes is an efficient utilization of the available bandwidth.
Subsidiary, but important transmission schemes within complex transmission schemes are digital transmission schemes. Therein, the information signal includes a bit stream, which can be particularly efficiently transmitted in the above described complex modulation signal. Practically, the bit stream and the complex modulated signal are divided into a plurality of symbols each comprising the same length. Each bit sequence of the same structure will be allocated to a predetermined symbol type having a unique amplitude and phase in the complex modulation signal. Such digital complex modulation schemes are well known in the prior art and provide the advantage, that the information included in the information signal can be transmitted with higher quality compared to analog complex modulation schemes.
Particularly of interest in the fields of digital complex modulation schemes are complex angle modulation schemes like phase shift keying (PSK) and frequency shift keying (FSK). Therein, the information signal including the bit stream is merely modulated into the phase of the complex modulation signal. Although these transmission schemes inefficiently utilize the available bandwidth, the main advantage is that the signal is more robust against non-linearities.
Complex demodulators for demodulating the above described complex modulation signal may work according to a plurality of technical principles. The most important complex modulator types are cross-differential-cross multiply demodulators (CDM), limiter-discriminator integrate demodulators (LDI) and zero crossing demodulators (ZCD). Whereas a CDM demodulator is highly complicated since it requires a lot of electronic components leading to high power consumption, a LDI demodulator is highly error prone. A good compromise could be provided by the ZCD demodulator combining the advantages of the LDI demodulator and the CDM demodulator. However, this is only given, if the amount of different symbol types in the complex modulation signal is sufficiently high.
The ZCD demodulator basically consists of a plurality of phase detectors and a reconstructing processor. Each of the phase detectors receives the complex modulation signal and provides its output to the reconstructing processor. There is one phase detector for each predetermined symbol type detecting whether the phase of the modulated complex signal is higher or lower than the phase of the predetermined symbol type. Based on this information, the processor determines whether the modulated complex signal turns clockwise or anti-clockwise in the complex area and reconstructs the whole bit stream of the information signal based on predetermined mathematics.
Theoretically the output of each phase detector is constant at least over the duration of the predetermined symbol type. However, practically in the receiver, the complex modulation signal is noisy. This makes the complex modulation signal jittering while turning. However, due to this jittering, the complex modulation signal crosses the phase of a predetermined symbol type forward and backward with very high frequency. This leads to to a plurality of very short pulses output by the respective phase detectors. These short pulses are glitch components and therewith noise in the received information signal. In other words, the jittering could affect and even distort the whole reconstruction processing after the phase deciders.